The invention relates to thermocouple devices used in measuring high temperatures. A thermocouple is defined as a device consisting of two dissimilar materials (usually metals) which when joined together generate a predictable voltage based on the temperature of their junction.
Work with carbon (carbon and graphite are used interchangeably) thermocouples has been recorded as early as 1881 continuing to the present. These early thermocouples evolved to the type described by Westbrook and Shepard, as in U.S. Pat. No. 2,946,385, whereby a graphite/graphite with an additive or doping agent thermocouple was made. The doping agent, boron, made a graphite of sufficient electrical valence difference that when joined with the undoped graphite a measureable Seeback voltage was created. Thermocouples have been described in Physiochemical Measurements at High Temperatures, 1959, Bockres, White, McKenzie, where the doping agent was clay. Thermocouples of this type suffer from inhomogenity, nonuniformity, and instability. Due to the low strength of the carbon materials, the physical size of these carbon thermocouples is much larger than typical metal thermocouples. Due to their physical size, they conduct heat away from the surroundings in which they are sensing.
A second approach to carbon thermocouples is described by Jamieson in U.S. Pat. No. 3,305,405. A material which yields graphite is pyrolized and deposited on a surface of boron nitride. A second layer of pyrolytic graphite which has seen a different heat history is then deposited. The different heat histories result in a sufficient electrical difference to generate a Seebeck voltage. This method of fabrication is difficult and batch-to-batch uniformity would be a problem.
Accordingly, an objective of the present invention is to provide a thermocouple which is stable and efficient for use in measuring high temperatures.
Another object of the present invention is to provide a thermocouple having a temperature sensing head is simple in construction and has a small mass to reduce conduction heat loss.